<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 
      320~375之间 font-size: 15px;
      375~414之间 font-size: 18px;
      414~480之间 font-size: 21px;
      大于480px font-size: 24px
    */
    /* @media (min-width: 320px) and (max-width: 375px) {
      .box { font-size: 15px; }
    }
    @media (min-width: 375px) and (max-width: 414px) {
      .box { font-size: 18px; }
    }
    @media (min-width: 414px) and (max-width: 480px) {
      .box { font-size: 21px; }
    }
    @media (min-width: 480px) {
      .box { font-size: 24px; }
    } */

    /* CSS层叠性 */
    @media (min-width: 320px) {
      .box { font-size: 15px; }
    }
    @media (min-width: 375px) {
      .box { font-size: 18px; }
    }
    @media (min-width: 414px) {
      .box { font-size: 21px; }
    }
    @media (min-width: 480px) {
      .box { font-size: 24px; }
    }

  </style>
</head>
<body>
  
  <div class="box">
    我是box
  </div>

</body>
</html>